Inheritance in Java

Java classes are arranged in a parent-child hierarchy in which the parent and child are known as the superclass and subclass, respectively. We’ll explore these concepts fully but not now. In Java, every class has exactly one superclass (a single parent), but possibly many subclasses. The only exception to this rule is the Object class, which sits atop the entire class hierarchy; it has no superclass. The declaration of our class in the previous example uses the keyword extends to specify that HelloComponent is a subclass of the JComponent class:

 public class HelloComponent extends JComponent { ... }  

A subclass may inherit some or all the variables and methods of its superclass. Through inheritance, the subclass can use those variables and methods as if it has declared them itself. A subclass can add variables and methods of its own, and it can also override or change the meaning of inherited methods. When we use a subclass, overridden methods are hidden (replaced) by the subclass’s own versions of them. In this way, inheritance provides a powerful mechanism whereby a subclass can refine or extend the functionality of its superclass. For example, the hypothetical spreadsheet class might be subclassed to produce a new scientific spreadsheet class with extra mathematical functions and special built-in constants. In this case, the source code for the scientific spreadsheet might declare methods for the added mathematical functions and variables for the special constants, but the new class automatically has all the variables and methods that constitute the normal functionality of a spreadsheet; they are inherited from the parent spreadsheet class. This also means that the scientific spreadsheet maintains its identity as a spreadsheet, and we can use the extended version anywhere the simpler spreadsheet could be used. That last sentence has profound implications, which we’ll explore all through. It means that specialized objects can be used in place of more generic objects, customizing their behavior without changing the underlying application. This is called polymorphism and is one of the foundations of object-oriented programming. Our HelloComponent class is a subclass of the JComponent class and inherits many variables and methods not explicitly declared in our source code. This is what allows our tiny class to serve as a component in a JFrame, with just a few customizations.